In U.S. Pat. No. 4,355,903, Sandercock describes a thin film thickness monitor, seen here in FIG. 1, in which a light beam 11 is successively reflected from a reference thin film 13 of varying optical thickness and then from a sample thin film 15 of unknown optical thickness. A polychromatic light source, such as an incandescent lamp 17, provides a light beam 11 directed towards the reference thin film 13 with a broad wavelength range spanning the visible and near infrared regions (0.4-3.0 .mu.m). The reference thin film 13 is a transparent dielectric film layer 19 in the form of a semicircular ramp or wedge formed on a flat semicircular substrate 21 in turn attached to a flat circular metal disc 23. The thickness of wedge 19 varies in a known manner with the angle of rotation .THETA. of the disc 23, preferably linearly, from an initial zero thickness up to about 3.0 .mu.m. A number of film and substrate materials, including silicon dioxide on silicon, are suggested. Preferably, the reference thin film 19 and substrate 21 will be chosen to be identical in material to the sample thin film 15 and substrate 25. The light beam 11 is imaged at nearly normal incidence (.THETA..sub.1 .ltoreq.15.degree.) as a spot 26 on the wedge 19. The reflected light 27 is then imaged onto the sample thin film 15 and reflected from there to a light detector 29 with a broad wavelength response, preferably, one with a flat response in the 0.4-3.0 .mu.m wavelength range of the light source.
In general, light incident on a thin film supported on an opaque substrate is reflected partly from the film-air interface and partly from the substrate-film interface. As a result of the interference that occurs between the reflected beams, the reflected intensity will show an oscillatory dependence on the wavelength of the incident light. The reflectivity R(.lambda.) has certain maxima at wavelengths .lambda..sub.max and minima at wavelengths .lambda..sub.min that depend on the optical thickness n.multidot.d of the thin film and on the incidence angle .THETA. of the light. In the case of successive reflections from reference and sample thin films, 13 and 15, whenever the optical thicknesses (and incidence angles) are substantially the same, the set of wavelengths strongly reflected by the reference 13 will correlate strongly with the set of wavelengths strongly reflected by the sample film 15. Whenever the optical thicknesses are not substantially equal, there will be no general coincidence between the wavelengths of maximum reflectivity for the reference 13 and those for the sample 15. Accordingly, the light intensity observed by the detector 29 after both reflections will be a maximum, whenever the optical thicknesses are equal, provided the incidence angles are also equal. In the Sandercock apparatus, the circular metal disc 23 holding the reference thin film wedge 19 and substrate 21 is attached to a motor shaft 31 of a motor 33 that rotates the disc 23 at a uniform rate, and thereby turns the reference thin film wedge 19 relative to the light beam 11, so that the optical thickness of the spot 26 on the wedge 19 illuminated by the light beam 11 varies, preferably linearly, with time. Accordingly, the thickness of the sample thin film 15 is determinable from the elapsed time between the start 35 of the wedge 19, seen in a top plan view in FIG. 1A, and the detection of maximum intensity. The surface of the disc 23 not covered by the wedge may be coated black in order to better determine the exact starting point 35 of the wedge 19 from a sharp change in reflectivity as the disc 23 rotates.
The above described apparatus is based upon a number of assumptions that are not always valid in every circumstance, and which therefore limit the applicability of the device in a number of important cases. A number of elements in the system require a flat spectral response over a broad range of wavelengths. However, most lamps do not provide illumination which is completely flat as a function of wavelength. Many of the more sensitive light detectors lack even a stable, D.C. response with respect to time, not to mention a flat response over the desired wavelength range. These problems give rise to substantial distortion and false peaks in the retrieved signal. Further, the technique assumes that the sample substrate itself has a flat reflectivity characteristic with respect to wavelength, yet most substrates preferably reflect some wavelengths to a greater degree than others, thereby contributing their own distortion to the signal.
Another assumption is that the wedge used for the reference thin film has a constant slope so that the film thickness is precisely known for a given rotational angle. However, even though the linearity of wedges can be made better than 2%, process variations may prevent greater linearity from being achieved. Further, the actual slope of even a linear wedge may not be known with precision because of uncertainties in the etch rate or other process parameters.
The technique also assumes that the true position of the maximum reflected intensity is precisely known. However, the maxima are fairly flat and so the measured peak position at the detector may be affected by small amounts of noise in the light source output, detector response or the like. Even when the peak position is known, for thin films with thicknesses less than about 150 or 200 nm the measured peak position begins to deviate from the peak position expected from interference effects, until at thicknesses on the order of tens of nanometers, the peak position is no longer representative of thickness. Accordingly, the device is applicable only for films having thicknesses of about 150 or 200 nm and greater.
There is another assumption that there are no intervening optical surfaces contributing to a spectral response in the light path other than the two thin films. However, in some situations it is necessary to make observations in a vacuum chamber, which necessitates coupling the light through a vacuum port. This may contribute one or more observed peaks in addition to the one intended for measurement. Alternatively, the sample may contain multiple thin films which cause multiple maxima at different wavelengths.
Despite these shortcomings, the original Sandercock device has had great utility and been highly successful for many years. However, as the semiconductor industry strives for even greater precision, uses thinner and thinner films and employs one film on top of another, its limitations will become more apparent and the need for improvements in thin film measuring equipment will become more critical.
Accordingly, it is an object of the present invention to provide a thin film thickness monitor and method of measuring which still gives an accurate sample thin film measurement even when the assumptions described above are not fully met, as for example, when the reference thin film thickness does not vary precisely linearly or when the sample substrate does not have a flat spectral response.
Another object of the invention is to provide a thin film thickness monitor capable of accurately measuring sample thicknesses of less than 200 nm.
A further object of the invention is to provide a thin film thickness monitor that can make measurements through additional optical surfaces, such as a vacuum chamber port.